Time, hght absorption

At the same time, the use of an excimer laser as a hght source (A. = 308 nm) precludes the detection of the absorption signal at A. = 380 nm which is observed with an excitation light at A. = 266 nm and ascribed to tetramethyldigermene. This discrepancy could be also explained by the relatively low yield of 28 when A. = 308 nm is used for the excitation due to the significant difference in the molar extinctions of 26 in these two spectral regions. 

As noted above, fluorescence emission often follows two-photon absorption. This one-photon process occurs at a higher frequency than that of the exciting laser. Usually the spectral separation between the frequencies is large, minimizing scattered hght problems. The method is highly sensitive, as shown below, and has been used in many TPE applications. Offering excellent time resolution (better than 10 s), it has so far been the only one used in kinetic studies. 

Both phase transitions can be triggered by optical pulses [16]. The crystal is excited by an 80 fs pulse in the region of its CT absorption at temperatures between 77 and 105 K, thus near the transition temperature. The phase transition is detected by the changes in the reflection spectrum of the crystal. It is found that the phase transitions occur with a delay of 500 to 800 fs. Evidently it requires this time for an optically-excited local CT state to be converted into a macroscopic ionic I or neutral N phase. Here, cooperative electron-electron and electron-lattice interactions are presumably the driving forces for these photoinduced phase transitions. This is demonstrated by the dependence of the transition on the hght intensity. The phase transitions are shown schematically in Fig. 12.8. For further details of this process, which is still not understood in all its aspects, we refer the reader to the original literature [16]. 

The electrical photo response to excitation with a weak laser pulse of 10 ps duration absorbed in the bulk of Si is measured as time-dependent voltage drop across the external resistor Rm. If the time elapsed after the laser pulse is short compared to the RC constant of the circuit, the measured signal can be interpreted as photovoltage. It can be interpreted as photocurrent if the elapsed time is long compared to the RC constant. Figure 4 shows the response of an n-Si electrode in the ns time window to the absorption of a laser pulse of 10 ps duration. The black shaded area in Fig. 3 illustrates the generation of electron-hole pairs by the incident hght inside of the Si material. 

By studying the shape of the whole time-dispersed photon distribution from a scattering medium the absorption and scattering coefficients of the medium can be evaluated. The same information can be obtained by employing a sinusoidally modulated CW laser beam and studying the phase-shift and the modulation contrast reduction in the scattered Hght [10.225]. This is basically the phase-shift method discussed previously in Sect. 9.4.4. Several modulation frequencies are needed to obtain the same information as in the conceptually simpler pulsed techniques. 

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